I seem to have a problem. I just replaced my alternator with a used one that works. The car starts runs, but after a while if i have my sound on and step on my brake my sound system goes out. The car will also not start after a few days if I leave the battery attatched overnight or while I'm at work or school. I have a power fc and when I put it on the battery voltage display its usually is around 13.7 or 13.8. This is really fustrating and I'm not sure what the problem is. Any suggestions would be appreciated.

 

How do you know it works?

If you can duplicate this a 100% with just the brakes then some where in brake wiring/sound system a wire is crossed. A brake light does not draw that much amps.


13.7 volts with no load is a meaningless test. You can wire up a bunch of D
batteries and come up with 13 volts. There would not be enough amps
to start your car though. Multimeter is only good to tell you that your
battery is dead. It will not tell you if your battery is good.

To test for a good car battery underload, fully charge it first for several hours.
Disconnet fuel pump fuse so the car does not start. Hook up multimeter
to battery set to read dc volts. Turn the car over for several seconds
(with out the car starting) you multimeter should not drop below
9.6 volts.


Before you do the draw test, have you installed anything recently
(alarm, stereo, fog lights, fuel pump, battery (loose battery cable
will cause a drain)

To test for a drain. turn everything off. Make sure if the door is open
it is not drawing current (tape your door switch down). Hook up your
multimeter (with the wires plugged in to read amps). Switch meter
to read mili amps. Disconnect negative or pos battery wire. Put the meter
leads between battery and battery cable. If you get a negative number switch
your mult meter leads. With everything off your reading should be around
50 miliamps. If it is a lot higher than that pull one fuse at a time and see
which fuse draws the most most amps. When you found the fuse that
draws the most amps, start looking for everything on that circuit.